Heat 2 tablespoons oil in a large skillet over medium. Add onion; cook, stirring often, until tender, about 6 minutes. Increase heat to medium-high. Add beef; cook, breaking into pieces with a wooden spoon, until no longer pink, about 5 minutes. Stir in taco seasoning sauce; cook, stirring often until beef is cooked through, about 3 minutes.
Whisk lime juice and remaining 1 tablespoon oil in a large bowl. Add lettuce; toss to coat. Serve topped with beef, tortilla chips, pico de gallo, avocado, and radishes.
